A corrosive substance is one that will destroy and damage other substances with which it comes into contact. It may attack a great variety of materials, including metals and various organic compounds, but people are mostly concerned with its effects on living tissue: it causes chemical burns on contact.

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The word 'corrosive' is derived from the Latin verb corrodere, which means 'to gnaw', indicating how these substances seem to 'gnaw' their way through flesh or other material. Sometimes the word 'caustic' is used as a synonym but 'caustic' generally refers only to strong bases, particularly alkalis, and not to acids, oxidizers, or other non-alkaline corrosives. The term 'acid' is often used, inaccurately, for all corrosives.

A low concentration of a corrosive substance is usually an irritant. Corrosion of non-living surfaces such as metals is a distinct process. For example, a water/air electrochemical cell corrodes iron to rust. In the Globally Harmonized System, both rapid corrosion of metals and chemical corrosion of skin qualify for the "corrosive" symbol.

Corrosives are different from poisons in that corrosives are immediately dangerous to the tissues they contact, whereas poisons may have systemic toxic effects that require time to become evident. Colloquially, corrosives may be called "poisons" but the concepts are technically distinct. However there is nothing which precludes a corrosive from being a poison; there are substances that are both corrosives and poisons.

Use of personal protective equipment, including items such as protective gloves, protective aprons, acid suits, safety goggles, a face shield, or safety shoes, is normally recommended when handling corrosive substances. Users should consult a safety data sheet for the specific recommendation for the corrosive substance of interest. The material of construction of the personal protective equipment is of critical importance as well. For example, although rubber gloves and rubber aprons may be made out of a chemically resistant elastomer such as nitrile rubber, neoprene, or butyl rubber, each of these materials has different resistance to different corrosives and they should not be substituted for each other.

In chemical uses, high chemical reactivity is often desirable, as the rates of chemical reactions depend on the activity (effective concentration) of the reactive species. For instance, catalytic sulfuric acid is used in the alkylation process in an oil refinery: the activity of carbocations, the reactive intermediate, is higher with stronger acidity, and thus the reaction proceeds faster. Once used, corrosives are most often recycled or neutralized. However, there have been environmental problems with untreated corrosive effluents or accidental discharges.